I suggest you forget about that female and use a different one. She sounds stressed and that may take a couple of weeks to fix. Isolate her, no exercise, just feed to condition and keep up with water changes. After a few days, flare her to a different color of the male, preferably a smaller female. If she flares, exercise regularly for a few days. Then try flaring to the same color as the male. Also do this for a few days. Let her gain full confidence. She should be in breeding mode by this time - she should flirt swim when ever she sees another betta.

Set up the breeding tank and everything. Use extra hideouts. Since she had been stressed, I suggest immediately releasing her just before dark ( she might become stressed again if floated too long). Keep an eye on her for the first half hour. Make sure she is flirting with the male. At night, without extra light, they should become inactive but can sense each other. Hopefully she will spawn the following morning.

I left them undisturbed over night and all day today and this evening I found her hiding under the IAL. She's pretty torn up so I've taken her out and she's gone quite pale just like when I first got her. She's more active in her own tank.

I was wondering if the pale colour of the male would be the reason why she's not interested? Would a brightly coloured male be more attractive for her?

Color preference could be a reason. But this could also be caused by stressed. Many bettas won't go full out when once stressed by a certain color. . . . by confidence I mean, a betta will fight that color - not just flare. Mentality. IME light colors, royal blue and red have rather low mentality - meaning after stressed, it takes them longer to be willing to fight for territory (against that color).

You have to make your female aggressive, willing to fight the male's color. Aggressive with high mentality is easier and faster to breed. Otherwise she will never want to breed.

Note: I'm not advising you to fight them. Flare them 30-60 minutes (or longer). If she is still flaring, she has mentality. And the long flaring sessions will eventually get her into breeding mode.